M2M communication is an important application for fifth generation mobile networks. However, fixed-location M2M communication poses significant challenges due to the large number of MTCDs and the diverse application requirements.
Due to the large number of MTCDs, uplink (“UL”) timing synchronization is one consideration in M2M system design. A common application scenario for MTC is to support a massive number of meters and sensors that are deployed at fixed locations and infrequently report only small amounts of data. Therefore, one of the central issues for fixed-location M2M design is to reduce the overhead associated with acquiring and maintaining uplink timing synchronization. One problem is that using current long term evolution (“LTE”) mechanisms requires that fixed-location MTCDs perform random access procedures to acquire uplink timing synchronization before each data transmission or upon expiration of an uplink timing alignment period, which sets a maximum time limit on uplink timing synchronization for a mobile network. Since this uplink timing synchronization procedure may have to be performed several times per second, there is a significant cost, both in terms of overhead and energy consumption. LTE uses a relatively short uplink timing alignment period (“TAP”), since it assumes that the user equipment (“UE”) will be moving and the UE's internal clock will drift relative to the base station (“BS”) timing element. One skilled in the art will appreciate that the term User Equipment (UE) is often used to refer to terminals, such as wirelessly connected handsets. In the context of the following discussion, M2M terminal devices will be considered as UEs despite the fact that a user may not typically directly interact with them.
In LTE, uplink timing synchronization is usually obtained by the base station sending a timing adjustment (“TA”) signal to the user equipment in response to a random access request, or by sending a TA update from the base station to the UE. Open-loop (“OL”) synchronization may also be used by the UE to deal with mobility issues. When the UE moves, it can adjust uplink timing to compensate for the movement based on downlink synchronization and its own internal clock. If the UE's internal clock does not drift, most of the timing alignment issues caused by UE mobility can be eliminated by this OL synchronization method. Thus, OL synchronization can reduce the need for TA update signaling from the base station. However, for MTC applications, since fixed-location MTCDs are usually inactive (i.e. sleep) for long periods of time to save energy, waking up only infrequently to transmit small amounts of data, they typically have internal clock drift over time. Hence, following such periods of inactivity, MTCDs will have lost the previous UL timing synchronization and will need to re-do the synchronization process to obtain a new TA value from the base station. Moreover, since LTE uses a relatively short uplink timing alignment period, requiring frequent updates of UL timing, the MTCD must conduct frequent synchronization procedures to request new TA values from the base station for each uplink transmission. All of this results in increased overhead and inefficient use of MTCD resources. Furthermore, since uplink transmission from the MTCD is infrequent, TA cannot be estimated frequently by the base station from uplink data transmission.